To begin this question, remember the basic form of a linear function, y=mx+b. Then, assign the variables y and x to the information given in your problem! Think about the independent variable (x) and the dependent variable (y). Does the amount of water impact the price you pay, or does the money impact how much water is used? If you think about it logically, the first proposition is the most likely. So if your x = amount of water used, and your y = the monthly cost, then you already have the information to make two points! The first would be that the cost of using 16 HCF is $34.31, and using our x and y values, we can make this coordinate point (16, 34.31). Do the exact same thing with the next piece of information! 27 HCF costs $52.46, which makes the point (27, 52.46). Now that you have those points, you can use the formula (Δy)/ (Δx), change in y - change in x. This will give you the 'm' (slope) of your line! Now you've created an equation for a line in the form y=mx+b, so all you have to do to find the cost of using 25 HCF of water is to plug that into the equation you've just made. Let me know if you have any more questions!